The term bodies, particles and/or similar impurities is taken in this description to signify all material which, when it is located in a liquid, comes to rest against a separation apparatus, for example a grid or a screen through which the liquid passes. An expression which occurs often for such material is "screenings", consequently, in the continuation of this description, the expression screenings will often be employed for the above-defined matter. The designation "solid substances" also occurs but will be employed in this description generally exclusively for screenings in which the liquid content has been reduced.
In, for example, water treatment plants, in process industries such as pulp factories or paper mills, in food industries, such as slaughter houses, etc., there is a need to be able to remove from liquids bodies and particles and/or similar impurities.
When the screenings have been displaced of out of the liquid, the liquid content of the screenings is high. The total solids is generally less than 5-8%. There is, therefore, a need in the art to be able to reduce the liquid content in the screenings in order to facilitate its continued handling. In total solids contents exceeding 20%, the continued handling of the screenings is substantially facilitated. In cases when the screenings are incinerated, for example in municipal heating plants, even higher total solids contents are naturally extremely desirable, since this entails improved energy yield on combustion.
A further need in the art is that the screenings be freed of soluble, for example biological compounds included in the screenings. This applies particularly to screenings sent to landfill deposition.
From Swedish patent No. 457 637 and German Gebrauchmuster No. G 8905963, there are previously known apparatuses for removing solid bodies and material from a flowing liquid. These documents describe an obliquely upwardly inclined conveyor screw installed in a channel with flowing liquid containing bodies of solid matter. The conveyor screw is disposed in a path which, in its lower region, forms a part-cylindrical screen surface. Above the water surface, the path is designed as a tube or casing surrounding the conveyor screw. In its upper end, the casing is provided with a discharge aperture. In the region ahead of the discharge aperture, the threads of the conveyor screw are of reducing pitch, whereby a certain compression and dewatering take place of the matter being upwardly transported. However, capacity in these apparatuses is extremely low.
Apparatuses of the above described type are employed for mechanical treatment of restricted flows of municipal waste water, treatment plants or for purifying (screening) polluted liquids, for example water within industries such as paper mills, food industries and slaughter houses.
The equipment in existence suffers from a plurality of well known drawbacks and functional problems.
Thus, the upward conveyance of matter which has accumulated on the screening surface is unreliable. When the conveyor screw is designed as a conveyor helix, the screenings are readily washed down through the centre hole of the helix. Another well known drawback in the employment of the conveyor screw with a mechanical centre shaft is that the screenings adhere to the mechanical shaft and to the helical thread and rotate together with these, which greatly obstructs the conveyance function.
It will readily be perceived that, as a result of these inconveniences, the capacity of the equipment is reduced at the same time as such equipment requires manual supervision. In order, to some extent, to improve the conveyance function, it is known in the art to limit the angle of inclination of the conveyor helix. However, this leads to bulky designs and constructions and consequentially increased costs.
A serious drawback in the prior art technique is that the diameter of the conveyor helix is determined by the maximum liquid flow for which the apparatus is to be dimensioned, since the diameter of the conveyor helix is determined by the radius of curvature of the screen surface. Hence, large liquid flows require large diameters of the conveyor helix, which entails unwieldy and expensive designs and constructions. In large variations of the flow, the average degree of efficiency in prior art apparatuses is low. This circumstance is the immediate reason why apparatuses of the type described here have only come into use in relatively limited maximum flows, seldom above 100-125 l/s.
German patent application 94113084.1 describes an apparatus with two cooperating screen surfaces, which implies that the above-described drawbacks are partly obviated. However, the apparatus described in the patent document suffers from the drawback that the increased screen surface is, in its effective extent, restricted unless the construction is made extremely space-demanding and expensive. The reason is that they, according to the description, are oriented parallel with the inclining screen/upward conveyance apparatus. The increased screen surface also requires that the supply channel be expanded in the area of the screen surface.
Known embodiments of apparatuses described here also function unsatisfactorily as regards the reduction of the liquid content in the screenings, since there is no possibility in such apparatuses for control and regulation of the size of the amount of liquid which is removed from the screenings, and thereby also the possibility of controlling and regulating the total solids in the screenings which leave the apparatuses. One known drawback is that discharge of the screenings out of the conveyor path takes place using force of gravity, in other words reliance is placed on the matter falling out through the discharge opening under the influence of gravity. This technique entails that it is not uncommon that the discharge opening becomes entirely jammed.
Hitherto, the only demonstrated method of attempting to control the degree of compression and thereby the dewatering efficiency is to vary the length of the "unthreaded" compression space ahead of the discharge opening, i.e. to increase or reduce the length of the "friction plug" which is formed by the conveyed matter. The difficulty in attempting, in this manner, to control the degree of dewatering is obvious, since, in practice, the quantity of screenings per unit of time varies dramatically (the matter "dries" more or less when remaining for varying periods of time during which it is subjected to compression).